<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
     <input type="text" value="手机">
    <script>
        let input=  document.querySelector("input");
        console.log(input.value);
        input.onfocus=function(){
          
          if(this.value==="手机"){
                this.value=" "
    
          }
        }
        input.onblur=function(){
          this.value="手机"
        }
    </script>
</body>
</html>